Optical power splitters are required in many types of optical communications networks, such as long haul networks, metropolitan networks, and access networks, as well as in optical multiplexers and switches of such networks.
Fused fiber couplers can be used as optical power splitters, and have been known for many years as described, for example in U.S. Pat. Nos. 4,666,234, 5,064,267, 6,031,948, and 6,643,433, and have also been used within more complex optical systems, see e.g. U.S. Pat. No. 6,031,948.
In modern fiber optic telecommunications, much reliance is being placed on the state of polarization of light signals. Typically the polarization of the signal is used to help direct the signal along the fiber optic network. Network components or devices which function based upon the polarization of the light signal include fiber optic polarization tunable filters, depolarizers, binary polarization switch/modulators, polarization division multiplexers and many other polarization related fiber optic components. Many of these devices require fiber optic variable polarization beam splitters and/or combiners.
A variable polarization beam splitter in which the optical power split ratio is dependent on the polarization state of the light beam, and where the power split ratio can be controlled by means of liquid crystal cells, is described in U.S. Pat. No. 5,740,288.
However, for applications where the polarization state of the light beam is not known, and where a variable power split ratio is required, the beam splitters described in any of the aforementioned patents are not suitable: either the power split ratio is fixed, or the splitter relies on the polarization state of the light beam. One such application where both capabilities, that is a variable power split ratio as well as polarization independence, are required is in fiber CATV broadcast networks. In such system, the service company usually has to deploy a number of optical power amplifier to make sure that each end user will get enough optical power. With a variable power splitter, the optical power could be variable deploy among these end users. In this way the power margin could be shared and many amplifiers could be saved. Because the power may be deployed close to end user side, and at many different locations, the input light beam polarization state is difficult to know, so that the polarization independence device becomes a key issue for such applications.
Consequently, it is necessary to develop a variable optical power splitter providing polarization independence.